How did Christ, being the Son of God, become man?
Christ the Son of God became man, by taking to himself a true body, and a reasonable soul,1 being conceived by the power of the Holy Ghost in the womb of the virgin Mary, of her substance, and born of her,2 yet without sin.3
Scripture proofs
The Westminster Assembly's proof texts (KJV). The bracketed numbers in the answer above mark the clause each set of references supports; expand a row to read the verses.
1 John 1:14; Matt. 26:38
And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
Then saith he unto them, My soul is exceeding sorrowful, even unto death: tarry ye here, and watch with me.
2 Luke 1:27, 31, 35, 42; Gal. 4:4
Luke 1:27 To a virgin espoused to a man whose name was Joseph, of the house of David; and the virgin’s name was Mary.
Luke 1:31 And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS.
Luke 1:35 And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.
Luke 1:42 And she spake out with a loud voice, and said, Blessed art thou among women, and blessed is the fruit of thy womb.
But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law,
3 Heb. 4:15; Heb. 7:26
For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.
For such an high priest became us, who is holy, harmless, undefiled, separate from sinners, and made higher than the heavens;
This doctrine across the Standards
Where the Confession and the Westminster Shorter Catechism treat the same matter, so the question can be studied alongside its parallel statements.
In the Westminster Confession
VIII.2 The Son of God, the second person in the Trinity, being very and eternal God, of one substance, …
The Son of God, the second person in the Trinity, being very and eternal God, of one substance, and equal with the Father, did, when the fulness of time was come, take upon him man's nature, with all the essential properties and common infirmities thereof, yet without sin: being conceived by the power of the Holy Ghost in the womb of the Virgin Mary, of her substance. So that two whole, perfect, and distinct natures, the Godhead and the manhood, were inseparably joined together in one person, without conversion, composition, or confusion. Which person is very God and very man, yet one Christ, the only mediator between God and man.
→ VIII.2 · Of Christ the MediatorVIII.3 The Lord Jesus, in his human nature thus united to the divine, was sanctified and anointed with the …
The Lord Jesus, in his human nature thus united to the divine, was sanctified and anointed with the Holy Spirit above measure; having in him all the treasures of wisdom and knowledge, in whom it pleased the Father that all fulness should dwell; to the end that, being holy, harmless, undefiled, and full of grace and truth, he might be thoroughly furnished to execute the office of a mediator and surety. Which office he took not unto himself, but was thereunto called by his Father, who put all power and judgment into his hand, and gave him commandment to execute the same.
→ VIII.3 · Of Christ the MediatorIn the Westminster Shorter Catechism
Q.21 Who is the Redeemer of God's elect?
The only Redeemer of God's elect is the Lord Jesus Christ, who, being the eternal Son of God, became man and so was, and continueth to be, God and man in two distinct natures, and one person, forever.
→ Open Q.21Q.22 How did Christ, being the Son of God, become man?
Christ, the Son of God, became man, by taking to himself a true body, and a reasonable soul, being conceived by the power of the Holy Ghost, in the womb of the virgin Mary, and born of her yet without sin.
→ Open Q.22